1. Field of the Invention
The present invention relates to a mobile radio communications system and in particular to a control technique of radio parameters in the mobile radio communications system.
2. Description of the Related Art
A mobile radio communications system, typically a code division multiple access (CDMA) radio communication system, is composed of a plurality of base stations each forming radio zones (cells or sectors) to provide a service area. A mobile station (MS) can move throughout the service area without interrupting communication with a given base station. Since the mobile station moves away from or closer to a base station, into a building or between buildings, or across boundaries between adjacent radio zones, a radio-frequency (RF) propagation condition will dynamically change. Such changing RF propagation conditions are likely to cause poor communication, especially when communication needs to be handed off from one base station to another. To improve the quality of communications, some handoff control techniques have been proposed.
Japanese Patent Application Unexamined Publication No. P2001-136558A discloses a handoff-parameter control technique using a status monitor installed at a location in an overlapping area of two adjacent radio zones. More specifically, each base station periodically transmits a pilot signal and the status monitor receives the pilot signal to monitor Ec/Io, where Ec is a received signal strength of the pilot signal and Io is a received interference strength. The conventional handoff-parameter control technique changes handoff parameters (e.g. IS-95 CDMA soft handoff parameters such as an add threshold T_ADD and a drop threshold T_DROP) based on the monitored Ec/Io at all times. Since the handoff parameters are controlled depending on Ec/Io in which Io may dynamically change, the handoff parameters can be rapidly set to appropriate values in response to varying loads of the system.
However, the conventional handoff-parameter control technique uses the status monitor installed only in an overlapping area of two adjacent radio zones. Accordingly the handoff parameter control can be performed using the monitored Ec/Io only when a mobile station is moving by the status monitor.
Japanese Patent Application Unexamined Publication No. P2001-136562A discloses a system for obtaining location information of a mobile station by providing a plurality of point communication terminals within each cell to allow accurate location detection of a mobile station. However, this conventional system is designed to obtain accurate location information of a mobile station, not to control radio parameters.